It has long been known to make reinforcing fibers of glass, ceramics, carbon and certain polymers such as aromatic polyamides, etc. It has also long been known to coat the fibers with a sizing composition that protects the surfaces of the fibers against abrasive damage and enhances the bonding of a polymer or other matrix to the surface of the reinforcing fibers. The bond strength between the matrix and the surface of the fibers is extremely important for the physical properties of the composite materials since the strength of the reinforcing fibers is almost always significantly greater than the strength of the matrix material. Once the bond between the matrix and the surface of the reinforcing fiber is broken, the matrix can slip away from the fiber thereby causing strain or failure of the composite product. Since the bonding of the matrix to the surface of the fiber depends largely on the area of the fiber surface, increasing the surface area will also enhance the bond strength between the matrix and the reinforcing fiber. Reducing the diameter of the reinforcing fibers increases its surface area, but also increases costs and makes the manufacturing and processing more difficult. To further enhance bonding to a surface it is known to nano-roughen the surface of a substrate using a laser as disclosed in U.S. Pat. No. 6,350,506. To increase the surface area of reinforcing fibers it has also been proposed to etch or make nano-sized depressions in the surface of the fibers as disclosed in COMPOSITES SCI. TECH., 2006, Vol. 66, p. 509.
Chopped strand reinforced products, such as chopped strand for thermoset or thermoplastic resins, usually comprises glass fibers and can also comprise carbon, ceramic or polymer fibers, alone or in combination. These products are typically made from pellets or other forms, of one or a mixture of polymers having the fibers dispersed therein. These pellets, etc., are typically made by feeding bundles of fibers containing up to several thousand fibers, typically having a length of less than approximately 3 mm to about 7 mm or even up to approximately 250 mm, into a compounding or extruding machine along with one or more polymers and heating with high shear mixing to plasticize the polymer(s) and disperse the fibers therein. The current technology for long fibers includes feeding of the strands of fibers into molten polymer or other material matrix material, followed by cutting and compression molding.
Fiber products used to make FRP typically have a sizing that normally contains a coupling agent such as one or more silanes and one or more film formers or binders, and can contain other ingredients such as lubricants, surfactants, dispersants, fillers, stabilizers, antioxidants, biocides, and others that are needed or preferred for particular applications. The sizing is usually applied to the fibers as an aqueous slurry, solution, or emulsion, but liquids other than water may also be used. The amount and type of bonding agent(s) used in the sizing on the fibers results in stronger fiber-to-fiber bonding in the bundles. This aids the fiber handling characteristics, but may not be good for later processing and may affect the final product characteristics. To achieve good feeding characteristics in the fiber bundles, important to the final users, a substantial amount of film former or binding agent commonly is used in the sizing composition that is coated on each fiber to prevent filamentation during storage, shipment and handling. Filamentation, the breaking down of the fiber bundles resulting in excessive small fiber bundles and individual fibers in the product, causes bridging in the feeding bin cones, and other fiber handling equipment resulting in costly scrap and downtime.
Once in the compounder and in contact with the polymer(s), it is desirable that the bundles separate into individual fibers and that the fibers disperse thoroughly in the polymer(s). The time and amount of mixing action to accomplish this has a practical limit, and because of the strength of the bonds between the fibers, high-shear mixing commonly is required to achieve an acceptable degree of fiber dispersion and wet out (coating of the fibers with the polymer or polymer mixture). This shear damages the fiber surface and breaks the fibers into shorter segments, and commonly still falls short of optimum fiber dispersion. As a result, the produced reinforced plastic parts commonly do not reach their full potential with respect to surface characteristics and physical properties. Most product and process development work in this area is aimed at addressing these problems and opportunities.
Potential cost reduction opportunities exist in the chopped fiber bundle manufacturing processes by preparation of fibers having larger diameters. The fiber bundles are made by pulling fibers from a plurality of orifices in a fiberizing bushing, usually from tips, nozzles, on tip plates of a plurality of fiberizing bushings, while the material is in a molten or plastic state, cooling the fibers, coating the fibers with water or other cooling liquid medium such as glycols, and then with the sizing mixture usually containing one or more binding agents, gathering the fibers into strands, chopping the strands into segments of desired lengths, drying the wet chopped strands in an oven, and sorting the resultant dry bundles to remove undesirable small bundles and individual fibers, lumps and fuzz clumps. Typical processes are described in U.S. Pat. No. 3,996,032. These types of processes produce chopped strand bundles having a wide range of diameters and containing a wide range of numbers of filaments, e.g. from just a few fibers to 4000 or more fibers per segment. Many dry chopped strand products have been produced with the above-described processes and are used in making a wide variety of fiber reinforced products, but as described above, to achieve substantial improvement in bonding strength to the matrix, something different than the heretofore developed and proposed solutions is needed.
It has been documented that long fibers, with lengths typically more than 12 mm, provide reinforced products displaying improved physical properties, as reported by Thomason and co-workers (Composites A 1997, 28, 277 and Composites A 2002, 33, 1641). However, processing of long chopped glass fibers using standard equipment is not feasible due to the damage that the fibers would sustain under the high shear mixing and difficulties in dispersing of these long fiber bundles. In contrast, short fibers are easy to process and disperse, but they do not provide optimum properties within the composite materials.
It has been proposed to prepare an epoxy thermoset resin, which incorporates a woven continuous filament fabric, in which a sizing package including colloidal silica is applied to the woven fabric prior to the incorporation followed by vacuum assisted resin transfer molding. See, for instance, Army Research Laboratory Report No. ARL-TR-3241 (July 2004), and R. E. Jensen, S. H. McKnight, Composites Sci. Tech., Vol. 66, Pages 509 to 521 (2006). However, a long felt need still exists for overcoming current limitations of the sized reinforcing fibers to achieve better physical properties in the fiber reinforced polymer composite products.